Nov., 1950
EXCHANGE BETWEEN ALUMINUM CHLORIDE AND CARBON TETRACHLORIDE 5275 [CONTRIBUTION FROM TEE
DEPARTWENT OF CHEMISTBY. UNIVERSITY OF WXSCONSM]
The Exchange Reaction between Aluminum Chloride and Carbon Tetrachloride1 BY CAROL HERRMANN WALLACE AND JOHN E. WILLARD
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Introduction Aluminum chloride has been widely used as a catalyst in many reactions of the Friedel-Crafts type2 and it has been postulateda that in some of these the initial step in the mechanism is a reAlCla*R+ AICL-. action of the type RC1 If such a reaction, and the reverse step, can occur i t must lead to exchange between the chlorine of the aluminum chloride and that of the organic chloride. In agreement with this prediction Fairbrother4has found that the chlorine in acetyl chloride will exchange with that in aluminum chloride and, further, that radioactive hydrogen chloride is liberated when the acylation of benzene is catalyzed by aluminum chloride containing radioactive chlorine. These observations are consistent with the postulated mechanism but might also be accounted for by other mechanisms. I n order to learn more about the mechanism of exchange of the chlorine in aluminum chloride with organic chlorides we have investigated its reaction with carbon tetrachloride. Carbon tetrachloride was chosen because it avoids side reactions such as the production of hydrogen chloride. The investigations were carried out with the reactants in a vacuum system, with aluminum chloride prepared in situ and with purified, degassed carbon tetrachloride. For the purpose of this discussion the formula AlCl, will be used as a formal designation of aluminum chloride. It has been shown by X-ray diffraction studies5 on a single crystal thab aluminum chloride exists as an ionic lattice in the solid state and by electron diffraction measurement# that it exists as the dimer AlzCls in the There is evidence that vapor phase up to 400'. i t may exist as the monomer' in some solvents and as the dimer in othersS8 There is no information available on its formula in carbon tetrachloride solution.
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3AgC1 AI -c 3Ag MC18 in an evacuated system. The silver chloride was prepared by addition of Mallinckrodt Analytical Reagent silver nitrate t o solutions of hydrochloric acid containing C P with a specific activity of 0.3 mc./g.Q It was heated with an excess of Baker and Adamson 30 mesh aluminum metal (cleaned by immersing in aqueous hydrochloric acid, rinsing in water and drying) until at about 450" a rapid production of aluminum chloride occurred. When moisture was occluded in the silver chloride its presence could be determined after the synthesis by freezing out and counting the equivalent amount of radioactive hydrogen chloride which was formed by hydrolysis of the aluminum chloride. Such tests showed that silver chloride which had been dried by heating at 160" for four hours retained occluded moisture equivalent t o about 1% of its weight but that silver chloride which was fused on the vacuum system before reaction was completely dry. Aluminum chloride prepared from the two types of silver chloride gave equivalent results. Unless otherwise indicated aluminum chloride prepared in situ was used because it was more reactive than material synthesized in a similar manner but transferred from ampoules to the vacuum system in a drybox in which only ordinary care had been taken t o dry the gas. Carbon Tetrachloride.-The carbon tetrachloride was Mallinckrodt Analytical Reagent "low sulfur" grade further puriiied by: (1) illuminating for 40 hours with a 1000-watt bulb at 6 inches while saturated with chlorine gas; (2) washing with sodium carbonate solution; (3) washing with water; (4) drying over phosphorus pentoxide. It was distilled from fresh phosphorus pentoxide through a 15-inch Vigreux column, just prior to use, the center cut at the boiling point being used. After introduction to the vacuum system it was degassed and distilled through phosphorus pentoxide. Preparation and Separation of Reaction Mixtures.-In a typical experiment aluminum was added to silver chloride, which had been fused under vacuum, by jarring the tubing shown at B of Fig. 1. The mixture was heated, and the resultant aluminum chloride was sublimed into E which was cooled with liquid air. Tubes B were then sealed off. Purified carbon tetrachloride, degassed by
Experimental Radioactive Aluminum Chloride.-The aluminum chloride used in this work was prepared by the reaction (1) Presented before the Division of Physical and Inorganic Chemistry at the September 1949 meeting of the American chemical Society at Atlantic City, N. J. (2) (a) N. 0. Calloway, Chcm. Rev., 17, 327 (1935). (h) A. E. Remick, "Electronic Interpretation of Organic Chemistry," John Wiley and Sons, Inc., New York, N. Y.,p. 477, 1949; (c) C. A. Thomas, "Anhydrous Aluminum Chloride in Organic Chemistry," Reinhold Publishing Corp., New York, N. Y., 1941. (3) H. Meerwein, Ann., 456, 227 (1927). (4) F. Fairbrother, J . Chcm. SOC.,603 (1937). (5) J. A. A. Ketelaar, C. H. MacGillavry and P. A. Renes, RcC. Iron chim., 66, 501 (1947). (6) K. J. Palmer and Norman Elliott, THISJOURNAL, 60, 1852 (1938). (7) T.J. Lane, P. A. McCusker and B. Columba Curran, i b i d . , 64, 2076 (1942). (8) .J. A,'A. Ketelaar, Z. Krisl., 90, 237 (1935).
IIp U
Fig. 1.-Vacuum
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system for preparing AlClS-CCl, reaction mixtures.
(9) Item S-6, U. S. Atomic Energy Commission Isotopes Catalogue (Aug. 1949).
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CAROLHERRMANN WALLACE AND JOHN E. WILLARD
three cycles of freezing, evacuation and thawing was distilled from D onto the aluminum chloride, and E, together with the manifold of sampling tubes IT,was sealed off from the system and thermostated a t the desired temperature. Samples were taken by distilling carbon tetrachloride from E into one of the tubes N, freezing the liquid at both E and the tube nith liquid air and sealing off the tube. The sample was washed with water before counting to remove any traces of aluminum chloride which might have been carried over in spray. Small accurately known amounts of water vapor were, in some cases, added t o the aluminum chloride before addition of carbon tetrachloride by opening the breakseal to a tube of water vapor P. About ten minutes after admission of the water the tube C was cooled with liquid air and the hydrogen chloride produced by reaction of the moisture with the aluminum chloride was removed from the system and counted. When there was no other source of moisture, the number of moles of hydrogen chloride removed was equal to the moles of water added, indicating that the water reacted rapidly and quantitatively with the aluminum chloride in preference to reacting with the glass walls of the system. In all the work reported here the vacuum system was degassed by evacuating while flaming to temperatures just below the softening point of the glass. Final pressures were about 1 micron. Silicone "high vacuum" stopcock grease was used. The total amount of aluminum chloride initially present in an experiment was calculated from the summation of the counts of radiochlorine recovered in all the organic and aqueous samples and residues, corrected for the difference in counting yield in the two media. At equilibrium distribution of the radioactive chlorine >99.gCh of the activity would have been in the carbon tetraehloride in all experiments reported here, since the mole fraction of aluminum chloride relative to carbon tetrachloride was always less than Therefore the counting rate per ml. in the first carbon tetrachloride sample divided by the counting rate per ml. which would have been observed if all of the activity had transferred t o the organic liquid was equal to the fraction of equilibrium distribution of radioactivity which had been achieved. For succeeding samples, account was taken of the fact that activity had been removed from the system when the earlier samples were taken and that the volume of carbon tetrachloride into which further activity was exchanging from the aluminum chloride became less with each sample removed. Measurement of Radioactivity.-Radioactive alumnum chloride and carbon tetrachloride were separated by washing with water, and the TTater and carbon tetrachloride layers were counted with a Technical Associates No. TA-BI J glass walled Geiger-Mueller counting tube which had an annular jacket of 10-ml. capacity to hold the solution around the sermtive portion of the tube. The counting efficiency for the 0.64 ;\lev. beta particles from C136 dissolved in water wa5 about 3%. The :elathe counting efficiency in carbon tetrachloride ~olutionv. as 82% of that in mater. In order to calculate the total quantity of aluminum chloride present in each experiment the specific counting rate of the chlorine in counts min.-l mg.-' was determined by accurately weighing about 10 mg. of the radloactive AgC1, dissolving it in aqueous ammonia solution and counting it in the solution tube. It is believed that the values for weights of aluminum chloride deduced from counting rates are accurate to within 5T.
Results and Interpretation Products of the Reaction.-When radioactive d i d aluminum chloi idz a t "concentrations" greater than rn.11. was heated for two hours or more in a sealed tube a t 300" with a large mole excess of carbon tetrachloride and the mixture was then washed with water, essentially
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all of the radioactive chlorine was nearly always found in the organic fraction. This was true for aluminum chloride which had been prepared in the absence of all moisture and also for that which had been exposed to some moisture. A fractional distillation with carriers indicated that all of the radioactive chlorine was present as CC14. Since no activity was found in the aqueous extract i t appears that all of the chlorine atoms of either dry or partially hydroxylated aluminum chloride can enter into the exchange reaction. The Reaction a s a Function of Temperature.Table I gives data from two experiments which show the type of results which were repeatedly obtained when purified carbon tetrachloride, dried by distillation through phosphorus pentoxide, was distilled in vacuum onto dry aluminum chloride cooled with liquid air, and samples were then distilled off both as soon as the mixture had been rapidly melted (-21') and a t later times after the mixture had stood a t higher temperatures. TABLE I EXCHANGE REACTION BETWEEN ALUMINUM CHLORIDE AND CARBON TETRACHLORIDE T e m p , "C
Total time elapsed
Total % exchange
3.3 X 10-3 mole AlCI, per liter of CCh - 9 10 min. 16 - 10 2.0 hr. 16 0 6.5 22 26 9.5 33 51 26 100
6.0 X 10-3 mole AIC13 per liter of CClr 5 min. 15
-21 0 28 28 28 28 45
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1 hr. 1.5 2.0 2 6 3 0 12
19 49 49 49 49
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It seems reasonable to explain the fact that a limiting value of transfer of radioactivity is reached a t each temperature by the hypothesis that the solubility of aluminum chloride a t the lower temperature is so low that only a relatively thin layer of the aluminum chloride particles has opportunity to enter into the solubility equilibrium and that complete recrystallization of the particles is very slow. If this is the situation the transfer of radioactivity will be limited to the aluminum chloride in this outer layer. When the radioactive chlorine in this layer has been replaced by inactive chlorine the exchange reaction will continue but no further transfer of radioactivity will be observed since the remaining radioactive chlorine on the inside of the particles does not have access to the carbon tetrachloride. At a higher temperature, on the basis of this hypothesis, a greater transfer of radioactivity will be
Nov., 1950
EXCHANGE BETWEEN ALUMINUM CHLORIDE AND CARBON TETRACHLORIDE5277
observed before the reaction appears to stop, because the solubility of the aluminum chloride is higher and a greater depth of surface is turned over in the solubility equilibrium. It seems probable that variations in the amount of activity exchanged a t a given temperature for different runs are due to variations in the particle size, and hence the surface to volume ratio of the aluminum chloride in the different experiments. The Effect of Moisture on the Exchange Reaction.-Exposure of the aluminum chloride surface t o water equal to 10 mole 7 0 of the aluminum chloride, when the latter is present a t a “concentration” of about 5 X loF3 m./l., seems to have no effect in decreasing the rate of the exchange reaction (Expt. (l), Table 11). Exposure to somewhat larger amounts of moisture stops the exchange reaction a t the lower temperatures studied (Expt. (2), Table 11) and decreases the rate a t the higher temperatures studied (Figs. 2 and 3). TABLE 11
5 q 40
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Time, hr.
Fig. 2.-Exchange of slightly hydroxylated AlC13 with CC& as a function of time: the curves refer to runs with three different samples of AlCla, each of which was exposed to some moisture during transfer in a dry-box.
EXCHANGE REACTION BETWEEN CARBON TETRACHLORIDE AND ALUMINUM CHLORIDE TO WHICH WATERHADBEEN ADDED Temp., “C.
Total elapsed time
Total yo exchange
Expt. (1) Mole AlC13, 1.2 X (4.5 X m./l.) Mole HzO added, 1.5 X Mole HCI removed, 1.5 X 10-6 -21 5 min. 11 0 1 . 0 hr. 22 25 1.5 50 25 2.0 53 25 2.5 52 44 3.0 73 100 6 100 Expt. (2) Mole AlCla, 1.6 X lo-‘ (5.3 X m./l.) Mole HzO added, 2.6 X 10-6 Mole HCl removed, 2.5 X loT6 -21 5 min. 0 0 1 .O hr. 0 28 1.5 0 55 2.0 7.5 55 3.0 15
The silver chloride used in Expt. (1) of Table I1 was fused under vacuum prior to synthesis of the aluminum chloride so i t is believed that the only water which came in contact with the latter was the 1.5 X mole which was added. Since the value given for moles of aluminum chloride is for the amount calculated by counting the radiochlorine in the aqueous extract a t the end of the experiment, the total moles of aluminum was that equivalent to this chlorine (1.2 X lo-‘ mole) plus that which was hydroxylated, as indicated by the hydrogen chloride collected (1/3 mole) or a total of X 1.5 X = 5 X 1.25 X mole, and the ratio of hydroxyl groups to chlorine atoms was 0.05/1.20 = 1/24.
T i m e , hr.
Fig. 3.-Exchange of slightly hydroxylated AlCt with CCla as a function of time: reaction of a single sample of AlCl3 a t two temperatures.
Within the limits of accuracy of this work this amount of water did not have any effect on the exchange reaction. The silver chloride used in Expt. ( 2 ) of Table I1 was not fused before synthesis of the aluminum chloride and therefore contained a small indeter. minate amount of moisture. It may be estimated from other experiments however that the aluminum chloride did not contain more than one hydroxyl group per 8 chlorine atoms. It should be emphasized that the average ratio of hydroxyl to chlorine in the reactant is, a t best, only an approximate criterion of whether reaction will occur a t low temperatures because the exchange reaction occurs only with the outer layers of the aluminum chloride particles. The ratio of hydroxyl to chlorine in these layers, after water is added to the particles, is undoubtedly much higher than the average ratio throughout the particle. The decrease in reactivity following addition of a given mole ratio of water may be expected to be less when the particle size of the aluminum chloride is low than when i t is high because in the former case the surface to mass ratio is larger and hence the depth of the hydroxylated skin on the particles is less.
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HERRMANN WALLACE
One possible explanation of results of the type illustrated in Expt. (2), Table I1 and in Figs. 2 and 3 is as follows: (a) an aluminum hydroxide surface layer completely protects the aluminum chloride from reaction a t the lowest temperature; (b) a t higher temperatures the pure aluminum hydroxide film is partially dissolved, or is partially eroded away by the increasing tendency for underlying aluminum chloride to dissolve, and chlorine exchange occurs; tc) as exchange occurs the transfer of radioactivity slows down and nearly stops as the result of deposition of non-radioactive aluminum chloride on the surface. A comparison of the rates of the reactiori at 26’ and those a t 50” in Fig. 2 has only qualitative significance m c e it was not possible t o reproduce quantitatively eithcr the total amount of aluminum chloride used in successive experiments or the surface area per mg. I n two experiments with all conditions identical except the surface area the oiie with the larger surface area would be expected to react the faster. On the other hand, in two experiments with the same surface area of aluminum chloride but different quantities the one with the smaller quantity would be expected to ShOW the greater fractional transfer of its radioactivity to carbon tetrachloride per unit time siiicc given fractional transfer in the latter case represents a smaller quantity transfer than in the former. Effect of Changing the AIuminum Chloride Surface by Sublimation.-Sublimation of aluminum chloride which had been exposed to moisture might be expected to rr&icc the surface which had been partially hydroxylated with n o t i hydroxylated aluminum chloride from the interior of the particles and thus make i t more reactive in the exchange reaction. This prediction was tested as ftrllows. The rate of transfer of radioactivity to liquid carbon tetrachloride from aluminum chloride which had been briefly exposed to slightly moist air was determined a t 30’. The carbon tetrachloride was then cooled to 0” to stop further exchange and was distilled at this temperature into an auxiliary reservoir surrounded with liquid air. LVhile the carbon tetrachloride was thus srpdrated the aluminuni
T l m e , hr
Fig. 4.-Effect of subiimdtion of ilightly hydroxy1,irctl AICIS011 the rate of cxchmgc with CCls at 30”.
AND JOHN
E. WILLARD
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chloride was partially sublimed from one part of the initial reservoir to another, This treatment may have increased the total surface area as well as exposing aluminum chloride which was less hydroxylated and turning over the inactive aluminum chloride which had been formed by exchange. Following the sublimation the carbon tetrachloride was poured back onto the aluminum chloride and the exchange reaction a t 30’ again followed. The results shown in Fig. 4 indicate a much faster reaction after the sublimation than before. Absence of Effect of Air on the Exchange Reaction.--A single experiment in which 9 mg. of aluminum chloride was prepared in situ on the vacuum line and then exposed for 15 min. t o air dried with phosphorus pentoxide, after which the air was pumped out and carbon tetrachloride distilled in through phosphorus pentoxide, showed that exposure of aluminum chloride to dry air does not decrease its reactivity in the exchange re:tction. Effect of “Concentration” of Aluminum Chloride on the Extent of Reaction.-Although 9’7100% exchange occurred when tubes containing aluminum chloride a t “concentrations” of 10 to lo-’ tn./l. in carbon tetrachloride were heated for two hours at loo’, “concentrations” of l o - $ m.11. and below gave only 1 to 10% exchange. Intermediate ‘concentrations” gave values ranging from 20 to 85%. Such a drop in exchange with decreasing ratio of aluminum chloride to carbon tetrachloride might result if : (1 ) small, approximately coriitant, aniounts of aluminum chloride were ad>orbed on the walls of the reaction vessel in such
